Normally in integrated amplifiers pre-out is before the volume control and directly behind the input selector and , if present, the RIAA pre-amp.
Sometimes buffered, often not.
In this case it means it is meant as the output where it is functioning as a pre-amp (pre-out) so you can use that output to drive active speakers directly and use the volume control as well as amplification for it.
So pre-out in this case has a different meaning as pre-out in integrated amps.
It is the same signal as the headphone out but different socket (RCA instead of TRS).
The RCA output might switch off when a headphone is inserted, or it might not ... depends on whether the RCA output is wired through the 6.3mm TRS socket (it has a switch function in it).